1. Field of the Invention
The present invention relates to a preferably oriented nanotwinned Au film and a method of preparing the same, and especially to a nanotwinned Au film composed of a plurality of nanotwinned Au grains stacked along a [220] crystallographic axis orientation in a thickness direction and a method of preparing the same.
2. Description of Related Art
The hardness and mechanical property of a metal material could be adjusted by the grain size of the metal. For example, some metal films with nano-grains or nanotwinned structures have higher hardness. In general, gold has a low hardness. When gold is used as an accessory, it could be easily deformed due to collision or extrusion. Thus, not only the aesthetic level of the gold accessory is reduced, but also the value of the gold accessory processing is decreased as well. Gold is also commonly used in jewelry-inlaying. However, since gold has low hardness; hence, the inlaid jewelry could easily fall off from gold. Accordingly, nanocrystalline Au grains or nanotwinned Au structures could be formed to increase the hardness of gold. Consequently, the surface hardness of the gold accessory or jewelry-inlaid gold could be improved to ensure wear resistance and to prevent the inlaid jewelry from falling off from gold. In addition, nanotwinned metals with nano-crystallinity could also be applied as the metal materials such as a through silicon via (TSV), an interconnection, a pin through hole, a metal wire (e.g., a copper interconnect), a circuit of a substrate. This could ensure the reliability of the electrical contacts and prolong the usage lifetime.
Gold also has a high conductivity nature. It is; thus, also a highly suitable metal for electrical connection in package structure. However, the Au grains in conventional electrical contacts made of gold materials lack specific crystallographic orientation. Since the Au grains at the surface of the electrical contacts are in random orientations; hence, the bonding process needs to be performed at a higher temperature or higher pressure. The excessively high temperature or high pressure could easily damage the elements of electronic products, such as a semiconductor chip. If the temperature of the gold bonding process is decreased, then, a higher pressure is required. In this way, the gold bonding process would be too complicated and necessitate expensive equipment. The overly high pressure could also affect the electronic product yield as well.
Conventional methods for preparing a nanotwinned Au film include the followings: (1) The nanowire plating method for preparing an Au nanowire with nanotwinneds; this method; however, cannot prepare a nanotwinned Au film. It is also not widely used due to its long processing time; (2) The annealing method for preparing a nanotwinned Au film; this method; however, cannot successfully prepare an Au film with high density of nanotwinneds and regular arrangement; (3) the alloy method followed by the annealing process for preparing a nanotwinned Au structure; this method; however, needs the use of alloy, which reduces not only the purity of the prepared gold but also the value thereof when used in gold ornaments; and (4) the copper doping method for increasing the hardness of gold; this method; however, forms a karat gold with decreased luster and purity than a pure gold, which results in its lower value than a pure gold.
Therefore, there is an urgent need to develop a novel Au film and a method for preparing the same, where the Au film could have a nanotwinned structure and a preferred orientation. Such novel Au film could then be used in the accessory industry to increase the hardness of gold accessory and improve the reliability of its associated processing. In addition, such novel Au film could also be used in the electronics industry. The property of the nanotwinned structure and a preferred orientation could avoid the conventional high-temperature and high-pressure process used for the preparation of electrical contacts. Thereby, the product yield of electronic products could be increased with reduced costs. Compact electronic products with high performance could also be prepared.